Systems to detect objects detect the distance, the relative velocity, the relative angle or the image information of objects in the target area. Systems are used for this purpose, which radiate waves and evaluate their echoes. In doing so, waves of different physical nature (e.g. sound waves or electromagnetic waves) and different wavelength (with electromagnetic waves, e.g. in the range of infrared or radar) are put to use.
Systems of this types are used in particular for regulating the distance and the driving speed of vehicles.
If a wave with the propagation velocity c is reflected on an object, e.g. on a vehicle running ahead at a distance a, after the running or transit time Δt=2a/c the system receives the reflected and in general damped wave e(t). Thus one can conclude or ascertain the distance of the object from the running time Δt, as long as the running time Δt is always smaller than the pulse repeat time TPW(n). Otherwise, ambiguity problems do arise—they are referred to as trappings or super-range readings. If the object detected by the wave moves with the relative velocity v relative to the measuring system, then the reflected wave received by the system shows a frequency shift by the Doppler frequency fD=2fsv/c. Thus one can conclude or ascertain the relative velocity v from the Doppler frequency fD.
In practice in motor vehicle applications, apart from trappings there is a further interference with an identical effect, namely if several vehicles with systems of this type meet each other so that pulses of the other systems of other vehicles are received by the system of the vehicle of interest.
Linear filtering is not suitable for suppressing such interferences, as on the one hand it only smooths the disturbances and on the other hand the spectral and power analysis already generally represents a linear filtering of minimum bandwidth.